Parts per million (ppm) and milligrams per liter (mg/L) are both used to express the concentration of a dissolved substance, such as a pollutant or mineral, in fields like environmental science and chemistry. Confusion often arises because they are frequently treated as interchangeable in water quality reports. However, they represent fundamentally different types of measurement: ppm is a proportional ratio, while mg/L is a volumetric measurement. Understanding this distinction determines their actual relationship in scientific contexts.
Defining Parts Per Million and Milligrams Per Liter
Parts per million (ppm) is defined as a ratio that indicates how many parts of a solute are present for every one million parts of the entire solution or mixture. It is a proportional unit, often representing a mass-to-mass ratio, such as one milligram per kilogram of solution. Because it is a ratio, ppm is also used as a volume-to-volume ratio when describing gas mixtures. This makes ppm a dimensionless value, meaning it relies on a comparison of quantities rather than physical units like length or volume.
Milligrams per liter (mg/L), conversely, is a physical unit of concentration that strictly adheres to a mass-to-volume relationship. It quantifies the mass of the dissolved substance, measured in milligrams (mg), present within a fixed volume of the total solution, measured in liters (L). This unit is a direct measure of density for the solute within the solution. For instance, a measurement of 5 mg/L means that five milligrams of the substance are contained within every single liter of the liquid.
The Specific Case of Water: Why They Are Equivalent
The common practice of treating 1 mg/L as equivalent to 1 ppm is a useful simplification that holds true almost exclusively for very dilute aqueous solutions, meaning those where the solvent is water. This equivalence is based on a specific physical property of water: its density. Under standard conditions, one liter of pure water has a mass of approximately one kilogram (1 kg).
A single kilogram is equal to 1,000,000 milligrams. When you dissolve one milligram of a substance in one liter of water, you are essentially placing one milligram of solute into one million milligrams of solvent. Therefore, 1 mg of solute per 1,000,000 mg of water is mathematically equal to 1 part per million parts by mass (1 ppm). This 1:1 relationship is extremely convenient for reporting concentrations in fields like drinking water quality and environmental monitoring.
Applications Beyond Water: When Density Matters
The convenient 1 mg/L \(\approx\) 1 ppm equivalence breaks down as soon as the solution’s density deviates significantly from that of pure water. This occurs in solutions that are highly concentrated, such as brines or industrial chemical baths, where the dissolved solids substantially increase the overall mass per liter. The density of the solution is no longer 1.0 kg/L, meaning a liter volume no longer approximates one million milligrams of solution.
The equivalence also fails completely when the solvent is not water, such as in oil, alcohol, or other organic chemicals. To convert mg/L to ppm in a non-aqueous solvent, a density correction factor must be applied. This involves dividing the concentration in mg/L by the solvent’s specific gravity. In these non-aqueous and concentrated scenarios, using the specific unit mg/L is scientifically more precise than relying on the general proportional term ppm.